Preamp My experience: The Fosi ZP3 Hiss Issue Explained

[PCAudioLover]

There have been reports of this all over the internet (Reddit, Facebook, YouTube, ASR forums, etc.), and I see a LOT of confusion and misinformation about the issue and some features revolving around this issue like the Bypass and High-Pass Filter (HPF) functionality, so I thought I would post a summation here for ZP3 customers and prospective customers alike so everyone gets the correct information. (I also thought I'd use the opportunity to list some improvements for the ZP3 near the end in case Fosi are listening.)

Here is a summation of the general issue:
"There is a high frequency hiss coming out of my speakers if Bypass mode is turned off."

As far as I can tell, this problem exists in ALL units of the Fosi ZP3 preamp. It's not a "per unit" issue that can be fixed with an RMA, and all examples of RMAs from customers hoping to fix this issue reported that the new units they received suffer from the exact same issue. So unless Fosi makes an update to this product (v2.0), I think it's in all units for now. Fosi can correct me if I'm wrong.

MORE DETAILS:

1. The ZP3's volume setting does not affect the hiss volume level or tonal character at all (so it doesn't get quieter when you turn the volume down, or louder when you turn the volume up.) The selected input source on the ZP3 does not effect the hiss either... in fact, you don't even need anything plugged in and it still hisses. It's just there. Someone else described it as "thermal noise", whatever that means.
2. There does seem to be a small difference in the volume of the hiss depending on if you're using the RCA or XLR outputs, but it's there on either output type.
3. If you turn Bypass mode on, the hiss goes away and the ZP3 is dead silent. (That means on the switch, the toggle is on the "BYPASS" label instead of the "OFF" label.)
4. In order to use the Tone Controls (Bass & Treble Adjustment) OR the High Pass Filter, you MUST have Bypass mode turned OFF. If you turn Bypass mode ON (the toggle on the "BYPASS" label), it will disable BOTH of these features. This is confusing many out there who seem to think the HPF still works when Bypass mode is turned on! The reason seems to be because some of Fosi customer support staff are confused and think the HPF still works when Bypass is turned on, along with a very misleading line in the product manual that leaves this little detail about the HPF out:
   
   THE MANUAL SAYS THIS:
   Bypass
   When this feature is enabled, the signal bypasses the bass and treble adjustment.
   
   There is no mention of the signal also bypassing the HPF! But I promise you, as confirmed by various tests and mic measurements, the HPF does NOT work when Bypass mode is turned on. If you move up the Fosi support chain, you'll also get confirmation of this. The following is an explanation straight from Fosi email support once they were pressed harder:
   
   "Currently, when the ZP3 is in Bypass mode, the HPF at the bottom is disabled. I have confirmed with our R&D team that the noise floor after the EQ circuit is inherently higher and cannot be changed. The HPF is actually processed through the OP chip NE5532."

WHAT THIS MEANS IN PRACTICE:

Like many of you, I purchased the ZP3 because it seemed like the perfect unit, giving me almost everything I wanted including (FINALLY) a high pass filter for proper bass management! 2.1 setups are the norm these days and so many people were asking for this feature, so props to Fosi for trying to implement. Unfortunately if you want to actually USE the HPF, you must have the Bypass turned off, which means you WILL have the hiss. Whether you can hear/notice the hiss or not depends on factors like your own hearing, how close you are to your speakers, and your other equipment like your power amp and the speakers themselves. But it WILL be there, and all you have to do is turn on your ZP3, turn bypass off, don't play any music, and put your ear up the your tweeter to confirm. Like many of you I've got a nearfield desktop setup, so unfortunately I do hear it.

But to be clear, the issue RESOLVED with the mix of equipment. You can't find "the right" power amp or speakers to ELIMINATE the issue with a perfect chain of product synergy here. It's noise being output by the ZP3 device itself.

HOWEVER, if you don't need to use the tone controls or the HPF, just make sure you have your ZP3 preamp in Bypass mode and the noise floor will be almost dead silent.

I wish Fosi were more transparent in their product specs when listing numbers for the ZP3's noise floor.. the listed specs seem to be for only when you disable these features on the device via Bypass mode. I find this misleading.

CHANGES NEEDED:

I'm hoping Fosi makes an update to fix the hissing/noise issue when using the High-Pass Filter. Version 2.0 of the ZP3!

Why did I say "when using the HPF" instead of "when using the HPF and/or tone controls"? Well, I'm honestly a bit surprised they are even messing around with these tone controls these days. Bass and Treble tone controls via unmarked knobs are such an antiquated feature. They are limited if not downright clumsy and don't give NEARLY enough control for what most people are after. (What frequency setting and bandwidth/range does each knob effect ? What's the dB gain adjustment range? ) These days, most people that are after tonal adjustment for either personal preference or room correction just use parametric EQ because it's so powerful and so ubiquitous now. Many DACs and streamers are providing EQ, or people are just using software like I am. Getting rid of tone controls on these products would save money and probably increase signal quality. It's a silly move in 2025/2026 if you ask me. Down with tone controls! But some of you probably disagree with me.

Look... the Fosi ZP3 gets 90% of the way there... and what it does get right, it gets very right. It's the right price, the right form factor (size is great... it even stacks with other gear), has some decent quality parts in most of the signal path, and it even has the perfect number and mix of input/output types on the back including features like the trigger. (Take note Schiit audio.) But the 10% they get wrong unfortunately REALLY cripples this product for a lot of consumers.

Here would be some very small changes Fosi could make to have made the ZP3 preamp almost perfect:

1. Remove the noise when the HPF settings are enabled.
2. Provide a few more steps/options for the HPF settings such as 60hz and 40hz. (MOST people will use the 80hz setting, but as a second choice more people use lower settings than higher. Who in the world is setting a 120hz crossover between their mains and sub? )
3. Remove the tone controls (or implement them in a way where there's less noise, or separate from the HPF circuit.)
4. Make the op amp swapping much easier (What a strange design choice: they made the op amps swappable by not soldering them in and adding the sockets, but then made them almost impossible to get to! Just put one of those removable glass tops on the unit like everyone else seems to be doing. What a miss!)
5. Replace that super rare and super annoying mickey mouse power plug (IEC C6 I think it is) and use a standard plug (IEC C14 I think?) so people can use their fancy audiophile power cables they already own, or if they need a longer power cord can get one more easily and cost effectively with the standard plug.
6. Make the power cord included with the unit a bit longer so people don't have to worry about buying a longer one in the first place.
7. PUT A VOLUME INDICATOR ON THE UNIT. (The most noted issue on the internet. Don't think I need to explain more.)

With these very, very simple tweaks, and keeping it at around the same price point, Fosi could have dominated the market with the ZP3 preamp for many years to come.

I'm still rooting for Fosi to get this right!